1. Field of Invention
Embodiments of the present invention relate generally to a system and a method for controlling debris under a web in a polishing system.
2. Background of Invention
In semiconductor wafer processing, the use of chemical mechanical planarization, or CMP, has gained favor due to the enhanced ability to increase device density on a semiconductor workpiece, or substrate, such as a wafer. As the demand for planarization of layers formed on wafers in semiconductor fabrication increases, the requirement for greater system (i.e., process tool) throughput with less wafer damage and enhanced wafer planarization has also increased.
An exemplary CMP system that addresses these issues is described in U.S. Pat. No. 5,804,507, issued Apr. 15, 1998 by Tolles et al., which is incorporated by reference in its entirety. Tolles et al. discloses a CMP system having a planarization system that is supplied wafers from cassettes located in an adjacent liquid filled bath. A transfer mechanism, or robot, facilitates the transfer of the wafers from the bath to a transfer station. The transfer station generally contains a load cup that positions wafers into one of four processing heads mounted to a carousel. The carousel moves each processing head sequentially over the load cup to receive a wafer. As the processing heads are loaded, the carousel moves the processing heads and wafers through the planarization stations for polishing. The wafers are planarized by moving the wafers relative to a polishing material in the presence of polishing fluid. The polishing fluid typically contains chemicals that aid in the removal of material from the wafer. The mechanical aspect of the polishing process is generally provided by abrasives disposed either in the polishing fluid (i.e., slurry) or disposed on the polishing material. After completion of the planarization process, the wafer is returned back through the transfer station to the proper cassette located in the bath.
One type of polishing material that may be utilized for chemical mechanical polishing is known as a fixed abrasive material. The fixed abrasive material comprises a plurality of abrasive particles suspended in a resin binder that is disposed in discrete elements on a backing sheet. As the abrasive particles are contained in the polishing material itself, systems utilizing fixed abrasive material generally use polishing fluid that do not contain abrasives.
Fixed abrasive polishing material is generally available in stick-down form but is often utilized in the form of a web. Generally, the web is supported by a platen having a central or working area where the polishing process is performed. The used and unused portions of the web that are not disposed on the platen are stored in a supply roll and take-up roll coupled to the side of the platen. As the web is consumed over the course of polishing a number of wafers, the web is advanced to place an incremental length of unused web in the working area of the platen. The used portion of the web leaving the platen is generally wound on a take-up roll that is disposed on the side of the platen opposite the supply roll.
Depending on the material to be polished, the unused portion of the web can be conditioned before entering the working area. Conditioning exposes the abrasive particles that are disposed in the abrasive elements that comprise the web. Conditioning is essential for scratch defect control when polishing soft materials. The web is typically conditioned by removing a layer of resin disposed at the surface of the abrasive elements to expose some of the abrasive particles disposed therein, and more importantly, to remove pad asperities and flatten the top of the individual abrasive elements while establishing a uniform height between the abrasive elements across the pad.
Both debris created during polishing, conditioning and contamination generated by other sources must be controlled to ensure good polishing results. For example, debris such as particulates from the conditioning or polishing process may become disposed on the backside of the web that is exposed between the supply roll and the top of the platen. Particles on the backside of the web are transported by the web as it advances. Some of these particles eventually become disposed between the web and the subpad. Since fixed abrasive webs are typically thin and flexible, particulate under the web may cause a corresponding xe2x80x9chigh-spotxe2x80x9d on the surface of the web. As light pressures used to hold the substrate against the web during polishing and the web and substrate have a low surface contact ratio, a high-spot on the web may create a large local contact force between the substrate and web as the substrate passes over the high-spot.
For example, on a system utilizing 2 psi of pressure to hold a patterned substrate against a web having an 18 percent contact ratio, the local pressure may be as high as 24 psi which is typically not great enough to cause a scratch. A particle under the web in such a system creates a high-spot that results in a force concentration that can greatly exceed 24 psi at the high-spot. This local force concentration results in scratching or other defects on the substrate""s surface. In some cases, one or more of the fixed abrasive elements over-lying the particle may be fragmented or sheared from the web due to the force concentration above the particle. Due to the periodic web advancement, a single particle on the web can cause the abrasive elements to be fragmented and/or sheared from multiple locations which forms a pattern of defects on the web. All the sheared and/or fragmented abrasive elements can create additional scratches on the substrate. Therefore, the effect of a single particle on the web can greatly amplify scratching issues during polishing. Additionally, global planarization of the substrate may be compromised due to a high rate of material removal caused by both the high-spot and the portions of the web damaged by the force concentration thereon when that portion of the web was positioned above the particle.
Therefore, there is a need for a system that controls debris under a web in a polishing system.
One aspect of the invention generally provides an apparatus for cleaning a backside of a web of polishing material. In one embodiment, the apparatus includes a platen having a support surface adapted to support the backside of the web and a web cleaner disposed on the platen adjacent the backside of the web. In another embodiment, an apparatus comprises one or more webs of polishing material, one or more polishing heads, one or more platens and a web cleaner. Each polishing head is adapted to retain the substrate against a respective web. Each platen has a support surface that supports the web from the web""s backside. The web cleaner is coupled to the platen and disposed against the backside of the web.
In another aspect of the invention, method for cleaning a web of polishing material is provided. In one embodiment, the method includes the steps of supporting a portion of the web of polishing media on a platen, advancing a portion of the web on to the platen, and cleaning a portion of a backside of the web.